Research has focused on plasma apolipoprotein (apo) composition and metabolism in normal and dyslipoproteinemic subjects. Patients with Tangier disease have an abnormal apoA-I (apoA-ITangier), which differs from normal apoA-I in amino acid composition, molecular weight, and in vitro and in vivo metabolism. ApoA-I absence, and autosomal recessive disorder, is associated with undetectable plasma concentrations of apoA-I. The interaction of apoA-I and apoA-II with plasma lipases is also being investigated. ApoA-II has been shown to activate hepatic lipase enzymic activity. Ongoing studies of apoE metabolism support the concept that most patients with type III hyperlipoproteinemia (HLP) have an abnormal apoE (E2). In addition, patients with type III HLP associated with a deficiency of apoE have recently been found, and represent a new cause for this from of HLP. In addition we have noted a much higher prevalence of apoE4 homozygosity in type V hyperlipoproteinemic patients, as compared to normals. Various dyslipidemic states have therefore been associated with specific apolipoprotein abnormalities.